It is conventional practice to make synthetic paper using synthetic pulp comprising short fibers of polyethylene. Such synthetic paper is made using polyethylene pulp with or without cellulose fibers. Such flexible polymeric synthetic substrates are used to make water-resistant cardboard, embossed paper, heat-sealing paper, battery separators, felt mats, hygienic absorbents and building materials. To meet the demands of various applications, many grades of polyethylene have become commercially available. These synthetic pulp products use polyethylenes of different physical properties. Polypropylene and polypropylene/polyethylene products are also known.
U.S. Pat. No. 5,047,121 to Kochar discloses a process for making synthetic paper containing at least 97 wt. % polyethylene on conventional continuous wet-lay papermaking equipment. The process includes the steps of: (1) preparing a pulp furnish comprising 97-99.5 wt. % polyethylene fibers and 0.5-3.0 wt. % polyvinyl alcohol binder fibers; (2) depositing the pulp furnish on the screen of a wet-lay papermaking machine to form a waterleaf sheet; (3) drying the resulting waterleaf sheet on heated drying cans having a drying profile wherein an initial drying phase is provided at a temperature between 200.degree. F. and 270.degree. F. to melt the polyvinyl alcohol fibers and a second drying phase is provided at a temperature between 190.degree. F. and 240.degree. F. to control stretch and elongation of the sheets; and (4) thermally bonding the dried sheet at a temperature between 250.degree. F. and 315.degree. F. to provide polyethylene paper. The thermal bonding can be accomplished with a calendar roll. The Kochar patent teaches that: (1) the strength of the synthetic paper can be tailored by varying the amount of polyvinyl alcohol fibers mixed into the polyethylene pulp; and (2) the porosity of the synthetic paper can be tailored by varying the bonding temperature.
In accordance with the teaching of the Kochar patent, the polyethylene pulp is fused to a degree dependent on the thermal bonding temperature. This results in a polyethylene paper which is suitable for the specific applications identified in that patent, i.e., filtration applications (e.g., vacuum cleaner bags) and battery separators. However, the low opacity of the resulting paper makes it unsuitable for use in high-quality printing. This is because the application of too much heat for a long duration causes the polyethylene pulp to flow to such a degree that it becomes increasingly translucent as it approaches a polyethylene film in structure.
Paper made of 100% synthetic fibers is useful as label paper. For example, the in-mold labeling of blow-molded plastic containers is less costly than conventional labeling methods in which labels with adhesive backing are adhered to the container in a separate step subsequent to blow molding. In-mold labeling eliminates this separate step, thereby reducing labor costs associated with handling of the adhesive-backed labels and capital costs associated with the equipment used to handle and apply adhesive-backed labels.
In accordance with conventional in-mold labeling of blow-molded plastic containers, labels are sequentially supplied from a magazine and positioned inside the mold by, for example, a vacuum-operated device. Plastic material is then extruded from a die to form a parison as depicted in FIG. 6 of U.S. Pat. No. 4,986,866 to Ohba et al., the description of which is specifically incorporated by reference herein. The mold is locked to seal the parison and then compressed air is fed from a nozzle to the inside of the parison to perform blow molding wherein the parison is expanded to conform to the inner surface of the mold. Simultaneously with the blow molding, the heat-sealable layer of the label of Ohba et al. is pressed by the outer side of the parison and fused thereto. Finally, the mold is cooled to solidify the molded container and opened to obtain a labeled hollow container.
For the sake of efficiency, it is desirable that the labeling of blow-molded containers be conducted continuously and rapidly. Also the labels to be applied during in-mold labeling should be sufficiently stiff that the automatic equipment used to handle the labels does not cause wrinkling or folding thereof. Conversely, the labels must be sufficiently elastic that they neither tear nor separate from the plastic container during flexing or squeezing of the latter.
A further disadvantage of conventional in-mold labels prepared from paper is that prior to recycling of the plastic container, the paper label must be removed using either solvent or mechanical means to avoid contamination of the recycled plastic material by small pieces of paper.
One prior art attempt to grapple with this recycling problem is disclosed in U.S. Pat. No. 4,837,075 to Dudley, which teaches a coextruded plastic film label for in-mold labeling of blow-molded polyethylene containers. The label comprises a heat-activatable ethylene polymer adhesive layer and a surface printable layer comprising polystyrene. The heat activatable adhesive substrate layer comprises a polyethylene polymer. Pigment or fillers are incorporated in the polystyrene layer to provide a suitable background for printing. An example of a suitable pigment is titanium dioxide and an example of a suitable filler is calcium carbonate. Preferably a layer is interposed between the adhesive substrate and the surface printable layer that comprises reground and recycled thermoplastic material used to prepare such labels. The label stock is prepared by coextrusion of the various label layers utilizing conventional coextrusion techniques. Separately applied adhesive is not employed.
The aforementioned patent to Ohba et al. teaches a synthetic label for in-mold labeling of blow-molded resin containers comprising a thermoplastic resin film base layer and a heat-sealable resin layer having a melting point lower than that of the thermoplastic resin base layer. The base layer has an inorganic filler, such as titanium dioxide or calcium carbonate, incorporated therein or incorporated in a latex coating thereon. The base layer may, for example, be high-density polyethylene or polyethylene terephthalate. The heat-sealable resin layer may, for example, be low-density polyethylene. The heat-sealable resin layer serves to firmly adhere the label to a resin container. In accordance with the preferred embodiment of the Ohba et al. label material for use on a blow-molded container made of polyethylene, four separate layers are joined together by coextrusion.
U.S. Pat. No. 5,006,394 to Baird teaches a polymeric film structure having a high percentage of fillers, for example, opacifying or whitening agents such as titanium dioxide and calcium carbonate. The fillers are concentrated in a separate filler containing layer coextruded with a base layer. The base layer may comprise polyolefins (for example, polyethylenes), polyesters or nylons. The filler-containing layer may comprise any of the same polymeric materials, but preferably comprises ethylene vinyl acetate coploymer. However, this film material is intended for use in disposable consumer products such as diapers.
In addition, U.S. Pat. No. 4,941,947 to Guckert et al. discloses a thermally bonded composite sheet comprising a layer of flash-spun polyethylene plexifilamentary film-fibril strand sheet in face-to-face contact with a layer of polyethylene synthetic pulp suitable for use in bar code printing. The layer of polyethylene synthetic pulp is formed by conventional wet-lay papermaking techniques.
The Dudley and Ohba et al. patents both disclose an in-mold label having a multiplicity of layers coextruded together. This complexity of structure raises the costs of manufacturing the respective in-mold label materials. Although there is no suggestion in the Baird patent that the film material disclosed therein would be suitable for use as in-mold label paper, if it were usable for that purpose it would suffer from the same disadvantage of being a relatively complex laminated structure and therefore relatively costly to manufacture. Likewise the patent to Guckert et al. discloses a laminated structure.